I've got all the other classes, all my buildings are lvl 3 but the alchemist, just finished Doubledoom's quest, and now I'm looking at the long list of quests, wondering which are actually worth doing.

I've got the big list of unlikely heroes quests, Ice and Fury, The hard dungeon quests (beat x with all the classes in building y) in each direction, as well as Halflings, Ho!

Considering how small all the rewards are for the effort involved, I'd rather focus on doing what it takes to get the monster classes, but the wiki is mute on how to go about doing this.

Perhaps, but now that I have little need for gold beyond preps and bank endgame, I'm mostly interested in unlocking the last major features of the game, and I didn't want to be playing a dozen bad race/class combos if there's no purpose.

I'd still recommend winning all the quests. Getting a full item pool lets you make all sorts of combos. As for the gold, doing Vicious, as a new player is quite costly. You might need to visit those quests to load up on cash.

We made an expansion and it is awesome. Really, you should check it out, especially if you're looking for some extra challenge.

The monster quests are probably the hardest quests in the game. Vicious is several steps above hard, almost a new game entirely. We don't have very many playthroughs of vicious because the original concept of playthroughs was to help beginners. That being said, I think that there is a desire for more advanced demonstrations and we should probably put some vicious content on that thread. I know that I personally would like to see some more vicious content from our resident experts.